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Invisible Captcha

Gem Build Status

Simple and flexible spam protection solution for Rails applications.

Invisible Captcha provides different techniques to protect your application against spambots.

The main protection is a solution based on the honeypot principle, which provides a better user experience since there are no extra steps for real users, only for the bots.

Essentially, the strategy consists on adding an input field 🍯 into the form that:

  • shouldn't be visible by the real users
  • should be left empty by the real users
  • will most likely be filled by spam bots

It also comes with a time-sensitive form submission.


Invisible Captcha is tested against Rails >= 4.2 and Ruby >= 2.3.

Add this line to your Gemfile and then execute bundle install:

gem 'invisible_captcha'


View code:

<%= form_for(@topic) do |f| %>
  <%= f.invisible_captcha :subtitle %>
  <!-- or -->
  <%= invisible_captcha :subtitle, :topic %>
<% end %>

Controller code:

class TopicsController < ApplicationController
  invisible_captcha only: [:create, :update], honeypot: :subtitle

This method will act as a before_action that triggers when spam is detected (honeypot field has some value). By default, it responds with no content (only headers: head(200)). This is a good default, since the bot will surely read the response code and will think that it has achieved to submit the form properly. But, anyway, you can define your own callback by passing a method to the on_spam option:

class TopicsController < ApplicationController
  invisible_captcha only: [:create, :update], on_spam: :your_spam_callback_method


    def your_spam_callback_method
      redirect_to root_path

Note that it is not mandatory to specify a honeypot attribute (neither in the view nor in the controller). In this case, the engine will take a random field from InvisibleCaptcha.honeypots. So, if you're integrating it following this path, in your form:

<%= form_tag(new_contact_path) do |f| %>
  <%= invisible_captcha %>
<% end %>

In your controller:

invisible_captcha only: [:new_contact]

invisible_captcha sends all messages to flash[:error]. For messages to appear on your pages, add <%= flash[:error] %> to app/views/layouts/application.html.erb (somewhere near the top of your <body> element):

<!DOCTYPE html>
  <title>Yet another Rails app</title>
  <%= stylesheet_link_tag    "application", media: "all" %>
  <%= javascript_include_tag "application" %>
  <%= csrf_meta_tags %>
  <%= flash[:error] %>
  <%= yield %>

You can place <%= flash[:error] %> next to :alert and :notice message types, if you have them in your app/views/layouts/application.html.erb.

Options and customization

This section contains a description of all plugin options and customizations.

Plugin options:

You can customize:

  • sentence_for_humans: text for real users if input field was visible. By default, it uses I18n (see below).
  • honeypots: collection of default honeypots. Used by the view helper, called with no args, to generate a random honeypot field name. By default, a random collection is already generated. As the random collection is stored in memory, it will not work if are running multiple Rails instances behind a load balancer. See Multiple Rails instances.
  • visual_honeypots: make honeypots visible, also useful to test/debug your implementation.
  • timestamp_threshold: fastest time (in seconds) to expect a human to submit the form (see original article by Yoav Aner outlining the idea). By default, 4 seconds. NOTE: It's recommended to deactivate the autocomplete feature to avoid false positives (autocomplete="off").
  • timestamp_enabled: option to disable the time threshold check at application level. Could be useful, for example, on some testing scenarios. By default, true.
  • timestamp_error_message: flash error message thrown when form submitted quicker than the timestamp_threshold value. It uses I18n by default.
  • injectable_styles: if enabled, you should call anywhere in your layout the following helper <%= invisible_captcha_styles %>. This allows you to inject styles, for example, in <head>. False by default, styles are injected inline with the honeypot.

To change these defaults, add the following to an initializer (recommended config/initializers/invisible_captcha.rb):

InvisibleCaptcha.setup do |config|
  # config.honeypots           << ['more', 'fake', 'attribute', 'names']
  # config.visual_honeypots    = false
  # config.timestamp_threshold = 4
  # config.timestamp_enabled   = true
  # config.injectable_styles   = false

  # Leave these unset if you want to use I18n (see below)
  # config.sentence_for_humans     = 'If you are a human, ignore this field'
  # config.timestamp_error_message = 'Sorry, that was too quick! Please resubmit.'

Multiple Rails instances

If you have multiple Rails instances running behind a load balancer, you have to share the same honeypots collection between the instances.

Either use a fixed collection or share them between the instances using Rails.cache:

InvisibleCaptcha.setup do |config|
  config.honeypots = Rails.cache.fetch('invisible_captcha_honeypots') do
    (1..20).map { InvisibleCaptcha.generate_random_honeypot }

Controller method options:

The invisible_captcha method accepts some options:

  • only: apply to given controller actions.
  • except: exclude to given controller actions.
  • honeypot: name of custom honeypot.
  • scope: name of scope, ie: 'topic[subtitle]' -> 'topic' is the scope.
  • on_spam: custom callback to be called on spam detection.
  • timestamp_enabled: enable/disable this technique at action level.
  • on_timestamp_spam: custom callback to be called when form submitted too quickly. The default action redirects to :back printing a warning in flash[:error].
  • timestamp_threshold: custom threshold per controller/action. Overrides the global value for InvisibleCaptcha.timestamp_threshold.

View helpers options:

Using the view/form helper you can override some defaults for the given instance. Actually, it allows to change:

  • sentence_for_humans
<%= form_for(@topic) do |f| %>
  <%= f.invisible_captcha :subtitle, sentence_for_humans: "hey! leave this input empty!" %>
<% end %>
  • visual_honeypots
<%= form_for(@topic) do |f| %>
  <%= f.invisible_captcha :subtitle, visual_honeypots: true %>
<% end %>

You can also pass html options to the input:

<%= invisible_captcha :subtitle, :topic, id: "your_id", class: "your_class" %>

Spam detection notifications

In addition to the on_spam controller callback, you can use the Active Support Instrumentation API to set up a global event handler that fires whenever spam is detected. This is useful for advanced logging, background processing, etc.

To set up a global event handler, subscribe to the invisible_captcha.spam_detected event in an initializer:

# config/initializers/invisible_captcha.rb

ActiveSupport::Notifications.subscribe('invisible_captcha.spam_detected') do |*args, data|
  AwesomeLogger.warn(data[:message], data)  # Log to an external logging service.
  SpamRequest.create(data)                  # Record the blocked request in your database.

The data passed to the subscriber is hash containing information about the request that was detected as spam. For example:

  message: "Invisible Captcha honeypot param 'subtitle' was present.",
  remote_ip: '',
  user_agent: 'Chrome 77',
  controller: 'users',
  action: 'create',
  url: '',
  params: {
    topic: { subtitle: 'foo' },
    controller: 'users',
    action: 'create'

Note: params will be filtered according to your Rails.application.config.filter_parameters configuration, making them (probably) safe for logging. But always double-check that you're not inadvertently logging sensitive form data, like passwords and credit cards.

Content Security Policy

If you're using a Content Security Policy (CSP) in your Rails app, you will need to generate a nonce on the server, and pass nonce: true attribute to the view helper. Uncomment the following lines in your config/initializers/content_security_policy.rb file:

# Be sure to restart your server when you modify this file.

# If you are using UJS then enable automatic nonce generation
Rails.application.config.content_security_policy_nonce_generator = -> request { SecureRandom.base64(16) }

# Set the nonce only to specific directives
Rails.application.config.content_security_policy_nonce_directives = %w(style-src)

Note that if you are already generating nonce for scripts, you'd have to include script-src to content_security_policy_nonce_directives as well:

Rails.application.config.content_security_policy_nonce_directives = %w(script-src style-src)

And in your view helper, you need to pass nonce: true to the invisible_captcha helper:

<%= invisible_captcha nonce: true %>

WARNING: Content Security Policy can break your site! If you already run a website with third-party scripts, styles, images, and fonts, it is highly recommended to enable CSP in report-only mode and observe warnings as they appear. Learn more at MDN:

Note that Content Security Policy only works on Rails 5.2 and up.


invisible_captcha tries to use I18n when it's available by default. The keys it looks for are the following:

    sentence_for_humans: "If you are human, ignore this field"
    timestamp_error_message: "Sorry, that was too quick! Please resubmit."

You can override the English ones in your i18n config files as well as add new ones for other locales.

If you intend to use I18n with invisible_captcha, you must not set sentence_for_humans or timestamp_error_message to strings in the setup phase.

Testing your controllers

If you're encountering unexpected behaviour while testing controllers that use the invisible_captcha action filter, you may want to disable timestamp check for the test environment. Add the following snippet to the config/initializers/invisible_captcha.rb file:

# Be sure to restart your server when you modify this file.

InvisibleCaptcha.setup do |config|
  config.timestamp_enabled = !Rails.env.test?

Another option is to wait for the timestamp check to be valid:

# Maybe in a before block
InvisibleCaptcha.timestamp_threshold = 1

# Before testing your controller action
sleep InvisibleCaptcha.timestamp_threshold


Any kind of idea, feedback or bug report are welcome! Open an issue or send a pull request.


Clone/fork this repository, start to hack on it and send a pull request.

Run the test suite:

$ bundle exec rspec

Run the test suite against all supported versions:

$ bundle exec appraisal install
$ bundle exec appraisal rspec

Run specs against specific version:

$ bundle exec appraisal rails-5.2 rspec


Start a sample Rails app (source code) with InvisibleCaptcha integrated:

$ bundle exec rake web # PORT=4000 (default: 3000)


Copyright (c) Marc Anguera. Invisible Captcha is released under the MIT License.

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